The present invention relates to cellular telephone systems, and more particularly, the invention relates to finding the position of a mobile station in a cellular telephone system.
Positioning in a cellular systems is a feature that is getting, more and more attention. There are a number of applications (Government, Operator and Commercial) that could use information about the current locations of mobile stations in a cellular telephone system.
To measure the location of a mobile station, either an external system, like the satellite based Global Positioning System (GPS), or a system, based on the cellular system itself, a Cellular Positioning System (CPS), can be used. A GPS system is undesirable because it requires extra hardware and cost. A CPS system is often more advantageous because it utilizes the existing hardware found in the cellular telephone system. A CPS positioning system can either be terminal based, where measurements are made in the mobile station or network based, where measurements are made in the network. The present invention is primarily directed to a network based CPS solution.
The position of a mobile station can be determined using measurements which could be time, angular, doppler measurements etc. Time of Arrival (TOA) measurements, measure the propagation time between a mobile station and a base station. Time Difference of Arrival (TDOA) measurements, measure the difference of the propagation time between the mobile station and two different base stations. Angle of Arrival (AOA) measurements, measure the angle to the mobile station relative to a certain direction from the base station. The measurements received from the base stations are then used to calculate the actual position of the mobile station. This procedure using well known geometric equations is called triangulation.
To find the actual position of a mobile, the measurements have to be mathematically converted into a position fix using a technique such as triangulation. The accuracy of the position fix is dependent on a number of parameters. An important parameter that will affect the accuracy of the position fix is the measurement accuracy. A good measurement accuracy yields a good accuracy of the position fix, and vice versa. Another is the measurement geometry, i.e., the relative position of the mobile station and the base stations involved in the positioning procedure. For a bad measurement geometry the accuracy of the position fix can be severely degraded. For example, if all three base stations BS1-BS3 of FIG. 1 from which measurements are made are located to either the left or right of the mobile station MS, then the accuracy can be degraded.
The relationship between the accuracy in the measurements (TOA, AOA, TDOA etc.) and the accuracy in the position fix is dependent on the locations of the base stations and the mobile station. The accuracy can be very poor for some configurations. This phenomenon is called Geometrical Dilution of Precision (GDOP). The GDOP is mathematically defined as the accuracy of the position fix divided by the accuracy of the measurements. If one is limited to measure from a subset or in a sequential order of base station candidates for some reason, GDOP and poor measurement accuracy can severely reduce the accuracy of the position fix with a bad choice of base stations. Moreover, if base stations other than those having the best signal to noise ratios (SNR) are used to make measurements, then the accuracy could also be similarly degraded.
Accordingly, there is a need for an improved positioning system which can overcome these problems.